1. Field of the Invention
The present invention relates generally to fuel cell devices with multiple fuel cells, and particularly to optimizing the size of the fuel cells based on their position.
2. Technical Background
The use of solid oxide fuel cells has been the subject of considerable amount of research in recent years. The typical components of a solid oxide fuel cell (SOFC) comprise a negatively-charged oxygen-ion conducting electrolyte sandwiched between two electrodes. Electrical current is generated in such cells by oxidation, at the anode, of a fuel material, which reacts with oxygen ions conducted through the electrolyte. Oxygen ions are formed by reduction of molecular oxygen at the cathode.
It is known that that at sufficient temperatures (e.g., about 600° C. and above), yttria stabilized zirconia YSZ (Y2O3—ZrO2) electrolytes exhibit good ionic conductance and very low electronic conductance. U.S. Pat. No. 5,273,837 describes the use of such compositions to form thermal shock resistant solid oxide fuel cells. Fuel cell devices of varies shapes are also known. For example, they may be square, round, rectangular or tubular. The air may be flowing in the direction of the fuel flow (co-flow), in the opposite direction (counter flow), in a perpendicular direction (cross-flow), or radially (radial flow). One common design approach is to utilize a planar fuel cell, where each electrolyte sheet corresponds to a single cell. However, the areas of the single sell that are hotter or correspond to higher reaction concentrations produce more power or current density.
U.S. Pat. No. 6,623,881 describes solid oxide electrolyte fuel cells which include an improved electrode-electrolyte structure. This structure comprises a solid electrolyte sheet incorporating a plurality of positive and negative electrodes of substantially equal sizes, bonded to opposite sides of a thin flexible inorganic electrolyte sheet. One example illustrates that the electrodes do not form continuous layers on electrolyte sheets, but instead define multiple discrete regions or bands, forming individual cells. These regions are electronically connected, by means of electrical conductors in contact therewith that extend through vias in electrolyte sheet. The vias are filled with electronically conductive materials (via interconnects).